1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotography method and a process cartridge used for the apparatus.
2. Description of the Related Art
In a conventional electrophotographic method, a coloring toner is developed to an electrostatic latent image manufactured by charging the surface of an image carrier and exposing the surface to produce a visible image, and the toner image is transferred onto a transfer paper or the like. Then the toner image is fixed by a heat roll or the like to form the image. Since an untransferred toner, an external additive and an electrical discharge product remain on the surface of the image carrier to which the transfer process was applied, it is necessary to remove them by using cleaning device before the next image forming process. Various methods such as a method using a fur brush and a magnetic brush or the like, and a method using an elastic cleaning blade have been used as cleaning device for removing transfer residual toner or the like. Devices for scraping the toner by scrubbing the image carrier by using the cleaning blade is generally used, because the means is simple and cheap. Although noncontact corona dischargers are widely used as devices which charge the image carrier, contact (or proximity) chargers have been used in recent years for the reasons such as space-saving, low-cost, the simplification of power supply and scarce ozone generation.
A method (DC bias applying method) of applying only a direct current voltage (DC) voltage to a charging member, and a method (AC bias applying method) of applying a oscillation voltage obtained by overlapping a direct-current voltage to an alternating voltage are used for the contact charging. In the AC bias applying method, the uniformity of the surface potential is easily obtained because the alternating voltage component removes unevenness of the charge, the charge is controlled at a predetermined voltage by the direct-current voltage component. On the other hand, in recent years, quality of the image forming apparatus of this type has been improving. For example, a polymerization method is used in order to achieve high image quality, since a diameter of toner can be decreased, toner can be sphered, and a particle size distribution can be sharper by the method. The reproducibility of dots formed on the image carrier can be improved by decreasing the diameter of the toner, and developing property and transferring property can be improved by sphering the toner.
However, the following problems exist in the conventional electrophotographic method. Friction resistance between the elastic blade and the image carrier is originally large, and thereby the elasticity blade cannot be slided. However, fine particles or the like added to the toner detach from the toner and exist between the elasticity blade and the image carrier to improve lubrication.
The amount of the fine particles between the blade and the carrier is affected by the formed image. Since the fine particles are not supplied in non image-forming cycle, damages such as the fluttering sound of the blade, inversion of the blade and nicks or abrasion of the cleaning edge or the like are caused particularly at high temperature and high humidity, in which the frictional force increases. As a result, the performance of removing toner/external additive/electrical discharge product is degraded. It is known that these phenomena are accelerated by a contact charging method, and the increase in the friction of the image carrier is particularly remarkable in a contact charging method of applying an AC bias. The increasement further accelerates damages such as fluttering sound of the blade, inversion of the blade, and nick or abrasion of the cleaning edge and it is difficult to maintain a constant cleaning performance for a long term.
It is known that a blade cleaning of the spherical toner manufactured by a polymerization method or the like is difficult, and the cleaning performance is more remarkably deteriorated than in the case of a conventional toner having an indefinite shape, particularly when the damage of the blade is exacerbated. Because of the deterioration of the cleaning performance caused by damage to the blade, it is difficult to elongate life of a process cartridge or the like which contains the cleaning blade.
A method is disclosed (for instance, see Japanese Patent Application Laid-Open (JP-A) No. 08-190252) which suppresses the fluttering sound of the blade and the damage of the cleaning edge. In the method, charging voltage components including at least the alternating voltage is stopped when the charging voltage is applied to non-image forming areas.
However, since it is necessary to apply a proper alternating current for securing the charging performance in image forming areas, the effect obtained by the method is not sufficient particularly when images are continuously formed.
Another method (for instance, see JP-A No. 08-194364) is disclosed in which the frequency of the alternating voltage component of the charging voltage is changed according to measured temperature/humidity in the image forming apparatus. However, it is still necessary to apply the alternating voltage element for securing the charging performance. Therefore, the obtained effect is not sufficient.
On the other hand, a cleaning blade is proposed in which a low friction layer primarily composed of a rubber and a resin is formed on the edge part of the cleaning blade (for instance, see JP-A Nos. 8-27227, 9-258632 and 11-24522).
In producing the cleaning blade, a low friction layer forming material is provided by mixing a silicone powder, a fluororesin powder, and a poly methyl methacrylate (PMMA) powder or the like in with binders such as a urethane rubber, a silicone rubber, a silicone resin, a fluorine rubber, a fluororesin, and a nylon. The low friction layer forming material is then coated on the image-carrier contact part (edge part) of the cleaning blade by a dipping method or the like to form a low friction layer. Therefore, the friction between the image carrier and the cleaning blade can be reduced, and the fluttering sound and inversion of the cleaning blade can be prevented.
However, although the cleaning blade with the low friction layer is effective at first, the low friction layer is worn by the friction between the image carrier and the blade. Thereby the effect is not expected to last long.
In addition, a powder or a liquid lubricant is added to the polyurethane rubber which is a material constituting the cleaning blade in many proposed methods for improving the lubricity (for instance, see JP-A No. 7-306616).
However, for instance, a polyurethane rubber cleaning blade obtained by adding the lubricant powder becomes hard to damage the image carrier in some cases. When a polyurethane rubber cleaning blade obtained by adding the liquid lubricant is used, the liquid lubricant moves onto the surface of the cleaning blade to stain the image carrier.
As shown in FIGS. 10 and 11, the applicant of the invention has already proposed (for instance, JP-A No. 11-161125) a cleaning apparatus which includes: a cleaning blade 129 whose tip face downwards and contacts with the surface of an image carrier 101; a housing 137 for storing the toner scraped from the surface of the image carrier 101 by the cleaning blade 129; a lower seal 130 for receiving the toner and guiding the toner to the housing 137; a toner accumulating member 131 which is nearer to the housing 137 than the cleaning blade 129 and the lower seal 130 are and which accumulates the toner at the tip of the cleaning blade 129.
Thus, the toner removed from the surface of the image carrier is accumulated in the area (hereinafter referred to as “area A”) whose periphery is defined by the cleaning blade 129, the lower seal 130, and the toner accumulating member 131. Consequently, the tip of the cleaning blade 129 is covered with the toner. External additives contained in the toner detach from the toner, and exist between the cleaning blade 129 and the photoreceptor. Thereby the external additives act as a lubricant, and the above problem can be prevented. Herein, numeral 101 designates a photoreceptor, and numeral 136 designates a long hole. Numeral 140 designates a conveying auger. θ is the contact angle between the cleaning blade 129 and the photoreceptor 101. T is the toner, and Pt is the pushing pressure from the toner T. G2 is the gap between the accumulating member 131 and cleaning blade 129. LAP is the part where the cleaning blade 129 and the accumulating member 131 overlap. R2 is the rotating direction of the photoreceptor 101. Numeral 111 designates a cleaning device.
However, as a result of further researches, the present inventors have found the following problems to be solved in the image forming method.
In the above method, the toner is temporarily accumulated near the tip of the blade, and the external additives contained in the toner detach from the toner. The external additives exist between the cleaning blade and the photoreceptor, and thereby the external additives act as a lubricant. Accordingly, the fluttering sound and inversion of the blade or the like are prevented. On the other hand, when the pressure of the toner at the tip of the cleaning blade 129 becomes excessive, the toner does not provide the lubricant effect but causes defective cleaning. Therefore, it is important to keep the amount and pressure of the toner at the tip of the cleaning blade 129 in the area A more uniform within the appropriate range.